+1, it would be awesome to filter at least by more of the standard tags, if not custom tags. I currently have filters set up on the toolbar for Genre and Year, which I use regularly (they are very helpful for cleaning up my genre tags quickly as well as, to some extent, identifying and correcting malformed dates such as Empty, 0, 198, etc.).
However, I have a custom tag that shows the Original Year unless it is blank, in which case it shows the Year. Having the ability to add this as a filter would be awesome.
Another aspect of filtering that would be intriguing would be create drop down menus from any displayed column heading, in much the same way as Excel does. Apologies if this idea is too far off from the original request, I will remove if that is the case.